1. Field of the Invention
This invention relates to a plasma processor, and more particularly, to a plasma processor for subjecting a semiconductor substrate to an etching or deposition process by means of a plasma stream.
2. Description of the Prior Art
Heretofore, a wet process which uses various solutions and large quantities of wash water has been employed for the etching of a thin film, such as a silicon dioxide (SiO.sub.2) film, a silicon nitride (Si.sub.3 N.sub.4) film, a polycrystalline silicon film, and an aluminum film, formed on a semiconductor substrate, or for the removal of a photoresist film formed thereon.
The process has involved many problems from the point of view of the use of large quantities of various chemicals, the safety of the operations, the technique and economy of foul solution disposal, the simplification and automation of the water processing stages of the work, etc. On the other hand, a dry process which exploits a gas plasma has recently come to be employed as a method for solving the above-mentioned problems of the wet process.
As a processor for semiconductor substrates which utilizes a plasma, there is generally used a structure such as shown in FIG. 1. In the illustrated apparatus, while a raw material gas for an intended processing, for example, freon or oxygen is being introduced into a quartz tube 1 through a gas introducing pipe 2 and is being exhausted through an exhaust pipe 3, the intra-tubular pressure is held at a value suitable for creating a discharge and for processing the substrate, for example, 0.5 - 2 Torr. By means of an oscillator 4 and a high frequency coil 5, a non-polar high frequency discharge is induced within the quartz tube 1 so as to generate the intended plasma 6 therein. The semiconductor substrates 8 to be processed are inserted in the tube 1 in advance in the state under which they are to be processed and they are supported therein on a holder 7. In this way, a selected operation on the substrates can be performed, for example, the removal of a photoresist film, or the etching of a silicon compound film, a polycrystalline silicon film, or an aluminum film.
It has been found, however, that the conventional plasma processor has the following disadvantages:
(1) Since the semiconductor substrate is placed in a high frequency electric field, it undergoes a temperature rise, and the photoresist film applied on the semiconductor substrate for local etching reaches its softening point and begins to flow.
(2) Since the semiconductor substrate is placed in the plasma of the high frequency discharge, electrons in the plasma are accelerated by the high frequency electric field and adversely affect the substrate. For example, the value of the surface resistivity of a silicon substrate increases, or the threshold voltage of a MOS transistor changes.
On the other hand, by utilizing a method of transporting a substance by means of a plasma, as disclosed in the specification of Japanese Patent Application Publication No. 38801/1970, a plasma generated in a plasma generating chamber may be transported onto a semiconductor substrate in a plasma processing chamber, to process a thin film on the substrate (Japanese Patent Application No. 74182/1972). According to this method, the semiconductor substrate need not be placed in the high frequency electric field, and hence, the problems of the heating of the semiconductor substrate ascribable to the high frequency electric field and the damage of the substrate ascribable to the acceleration of electrons in the plasma do not occur.
As a result of many experiments conducted for the performance of this method, however, it has been revealed that where the degree of vacuum in the plasma processing chamber is inferior, the plasma within the processing chamber recombines into neutral atoms, so that the efficiency of the plasma transportation lowers conspicuously. This invention is particularly directed to the solution of such a problem.